Memory storage capacity has been enhanced by fabricating dual storage node nonvolatile semiconductor memory cells that are able to store two bits of data in each memory cell. Such cells store information in a charge storage layer with charge storage locations at opposite sides of a control gate. If the charge storage layer is continuous between the storage locations, there is a potential problem of charge diffusion between the storage locations resulting in problems of data retention and reliability. The charge storage locations can be separated in a dual storage node device by utilizing an undercut structure such as disclosed in U.S. Pat. No. 6,861,307. Such a device, however, results in an undercut feature that is difficult to fill easily and reliably without leaving an air void. Additionally, especially as device dimensions shrink, it is difficult in such a structure to achieve the desired effective oxide thicknesses in the storage locations and in the gate insulator separating the storage locations.
Accordingly, it is desirable to provide a nonvolatile semiconductor memory device having isolation between dual storage nodes. In addition, it is desirable to provide a semiconductor memory device having enhanced data retention. It is also desirable to provide a nonvolatile memory device that is programmable by channel hot electron injection and that can be erased by Fowler-Nordheim tunneling. It is also desirable to provide methods for fabricating an undercut structure nonvolatile memory device having improved data retention. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.